1. Field of the Invention
The present invention relates to a liquid crystal driving device to drive a liquid crystal panel of a liquid crystal display device.
The present application claims priority of Japanese Patent Application No. 2006-069691 filed on Mar. 14, 2006, which is hereby incorporated by reference.
2. Description of the Related Art
Conventionally, a liquid crystal display device has a problem of a phenomenon in which an image leaves a trail when moving images are displayed. This phenomenon is caused by a delay in response of a liquid crystal element. To prevent this phenomenon, so-called overshooting driving is used in which a voltage to be applied to a liquid crystal element making up a liquid crystal driving circuit is controlled so as to be a voltage being higher than voltage to be applied when a still image is being displayed or a voltage being lower than a voltage to be applied when the still image is being displayed.
FIG. 9 is a diagram showing an example of configurations of a liquid crystal display device of a conventional technology. The liquid crystal display device, as shown in FIG. 9, chiefly includes a frame memory 1, an overshooting control section 2, an LUT (Look Up Table) 3, a timing control section 4, a gate driver 5, a source driver 6, a gray-level value setting section 7, and an active matrix type of liquid crystal panel 8.
The frame memory 1 stores image input signals for every frame according to a clock signal and outputs the signals with a time delay corresponding to one frame period. The overshooting control section 2 selects an overshooting gray-level value stored in the LUT 3 in a manner to correspond to a gray-level value of one past frame of an input signal and a gray-level value of a current frame and outputs the gray-level value according to an input signal and a signal output from the frame memory 1. The LUT 3 is made up of a table-like memory and stores data on overshooting gray-level values corresponding to one past gray-level value and a current gray-level value for every pixel of the liquid crystal panel 8. The timing control section 4 drives the gate driver 5 and the source driver 6 with timing of inputting of a signal according to an output from the overshooting control section 2. The gate driver 5, in response to driving by the timing control section 4, scans gates of a driving TFT (Thin Film Transistor) for each row of pixel lines through address lines in a vertical direction. The source driver 6, in response to driving by the timing control section 4, scans sources of a driving TFT for each column of pixel lines through data lines in a horizontal direction. The gray-level value setting section 7 sets a gray-level voltage for each data line supplied by the source driver 6 according to the operation timing of the source driver 6. The liquid crystal panel 8 displays an image corresponding to an input signal, according to scanning in vertical and horizontal directions, by driving of a pixel connected to a driving transistor mounted at every point of intersections of the address line and data line according to a supplied gray-level voltage.
In the liquid crystal display device shown in FIG. 9, the overshooting control section 2, when an n-th frame input signal and (n−1)-th frame input signal supplied with a time delay corresponding to one frame period by the frame memory 1 are input into the overshooting control 2, outputs, by referring to data stored in the LUT 3, data on an overshooting gray-level value corresponding to one past gray-level value and a current gray-level value for every frame, to the timing control section 4. The timing control section 4 controls operating timing of the gate driver 5 and source driver 6 and supplies data on an overshooting gray-level value output from the overshooting control section 2 for every frame to the source driver 6. This causes a driving transistor corresponding to each pixel formed at every point of the address line and data line in the liquid crystal panel 8 to be sequentially selected according to scanning by the gate driver 5 in a vertical direction and by the source driver 6 in a horizontal direction and to get into an operating state and each pixel to be driven via the driving transistor by a gray-level voltage corresponding to the overshooting gray-level value fed from the gray-level setting section 7 and, as a result, a desired image is displayed on the liquid crystal panel 8.
FIG. 10 is a table showing an example of data in the LUT 3. The LUT 3 contains data stored in the table-like memory which shows overshooting gray-level values corresponding to combinations of each of gray levels including one past gray level and a current gray level each being made up of 0 to 255 gray levels. The LUT 3 shows that, for example, if one past gray-level value is 128 and a current gray-level value still remains 128, the value 128 being the same as the one past gray-level can be provided as the overshooting gray-level value as it is, however, if the one past gray-level value is 128 and a current gray-level value is 64, a value 50 being lower than the current gray-level value should be provided as the overshooting gray-level value, and if the one past gray-level value is 128 and a current gray-level value is 192, a value 200 being higher than the current gray-level value should be provided as the overshooting gray-level value.
FIG. 11 is also a diagram showing another example of configurations of a liquid crystal display device of the conventional technology. The liquid crystal display device shown in FIG. 11 includes a frame memory 1, an overshooting control section 2, an LUT 3, a timing control section 4, a gate driver 5, a source driver 6, a gray-level value setting section 7, a liquid crystal panel 8, and an FRC (Frame Rate Controller) 9. Components other than the FRC 9 in the liquid crystal display device shown in FIG. 11 are the same as those in the liquid crystal display device in FIG. 9. The FRC 9 is configured to change configurations of gray-level values output from the source driver 6 without changing the number of gray levels of an input signal by performing a thinning-out operation on frames in gray-level driving to the source driver 6 to generate halftones. If the thinning-out of frames in the gray-level driving operations is, for example, a thinning-out of frames for ⅓ halftones, the FRC 9 can generate (N−1) gray levels, as a gray level to be output from the source driver 6, which is different by one gray level from N gray levels, by repeatedly exerting control with different timing sequentially for every 3 frames so that N gray levels are made to be (N−3) gray levels, as the gray level to be output from the source driver 6, by performing a frame thinning-out operation once for every three times. Similarly, the FRC 9 can generate (N−2) gray levels, as the gray levels to be output from the source driver 6, which are different by two gray levels from the N gray levels, by repeatedly exerting control with different timing sequentially for every 3 frames so that N gray levels are made to be (N−3) gray levels by performing a frame thinning-out operation twice for every three times by using a method of thinning-out of frames for ⅔ halftones.
Thus, in the liquid crystal display device shown in FIG. 11, halftones can be set as part of the output gray levels by using the FRC 9 having a function of generating halftones. To achieve this, an increase in the number of outputs from the source driver does not occur and, therefore, a rise in costs for the source driver 6 can be avoided.
FIG. 12 is a diagram showing a first example of an overshooting driving method applied to a conventional liquid crystal display device. In FIG. 12, a relation between a liquid panel applied voltage to drive a liquid crystal panel and transmittance through a liquid crystal panel (panel transmittance) is illustrated. Here, the panel transmittance denotes a ratio of light from a backlight transmitted through a panel and of outputting to a front of the panel and, if luminance of the backlight is constant, brightness of a screen is determined by the panel transmittance. Moreover, in FIG. 12, the applied displaying voltages V0, . . . , Vn represent a range of liquid crystal applied voltages in which the liquid crystal panel is driven to obtain brightness of a liquid crystal panel suitable to displaying, which is set according to a specified step (gray level) determined by the number of bits of input signals.
In the case of the overshooting driving method shown in FIG. 12, the applied displaying voltages having a range of V0, . . . , Vn corresponding to the range from its highest gray level to its lowest gray level are the same as the voltages containing corrected voltages for the overshooting driving. Therefore, in the case of transition from a gray level to the highest gray level or to the lowest gray level, there exists no voltage required for the overshooting driving to be added to or subtracted from the applied displaying voltage.
Thus, in the conventional liquid crystal display method shown in FIG. 12, since the applied displaying voltages having a range of V0, . . . , Vn corresponding to the range from the minimum of a panel transmittance to its maximum is the same as the voltages containing corrected voltages for the overshooting driving, in the case of the transition to its highest gray level or its lowest gray level for displaying, the overshooting driving was impossible.
FIG. 13 is a diagram explaining a state of changes in voltages applied to a liquid crystal panel at time of transition of gray levels when applied displaying voltages vary in the conventional liquid crystal driving method shown in FIG. 12, in which states of inverted driving for AC (alternating current) driving are shown on a positive polarity side all together. In FIG. 13, the mark “C” shows a state, as an example, in which a voltage applied to a liquid crystal panel is lowered in a manner to correspond to a decrease in a gray-level value when the applied voltage is at an intermediate level. This shows that, when the applied displaying voltage is lowered from its maximum value Vn to its intermediate value Vx in order to correspond to a decrease in a gray-level value, by performing an overshooting driving operation in which a voltage being lower than the voltage Vx is applied as shown by a solid line in FIG. 13, the voltage to be applied to the liquid crystal panel is set to be the desired displaying voltage Vx during a lapse of one fresh rate (frame rate) period.
On the other hand, the mark “A” shows a case in which a voltage to be applied to a liquid crystal panel is changed to the maximum value of an applied displaying voltage. When the applied displaying voltage is increased from its intermediate Vy to its maximum value Vn to correspond to changes in gray-level values, since there is no voltage on which the overshooting operation is to be performed, only the change in the voltage to be applied to the liquid crystal panel from Vy, . . . , Vn occurs. Due to this, the change in the voltage to be applied to the liquid crystal panel is not complete during the lapse of one fresh rate and, as a result, a remaining change in the voltage to be applied to the liquid crystal panel occurs during a subsequent refresh period. Therefore, when images are changed in displaying of moving images, a trail leaving phenomenon in which an image leaves a trail occurs. Moreover, the mark B shows the case in which the applied displaying voltage is changed to be the minimum value and, when the applied displaying voltage is lowered from its intermediate voltage Vx to its maximum value V0 to correspond to changes in gray-level values, since there is no voltage on which the overshooting operation is to be performed, only the change in the voltage to be applied to the liquid crystal panel from Vx, . . . , V0 occurs. Due to this, the change in the voltage to be applied to the liquid crystal panel is not complete during the lapse of one fresh rate and, as a result, a remaining change in the voltage to be applied to the liquid crystal panel occurs during a subsequent refresh period and, therefore, when a moving image is displayed, as in the case shown by the mark A, a trail leaving phenomenon in which an image leaves a trail occurs.
FIG. 14 is a diagram showing a second example of the overshooting driving method applied to the conventional liquid crystal display device. In the overshooting driving method shown in FIG. 14, a voltage corresponding to one gray-level value in the lowest level out of the applied displaying voltages V0, . . . , Vn and a voltage corresponding to one gray-level value in the highest level out of the applied voltages V0, . . . , Vn are assigned to voltages to be used only for overshooting driving, and the intermediate voltage range, except for voltages to be used only for overshooting driving, is used as displaying voltages.
In the conventional liquid crystal driving method shown in FIG. 14, it is possible that the overshooting driving operation is performed in a manner to correspond to all voltages within the applied displaying voltages, however, as a result of assigning parts of the applied voltages range to voltages to be used only for overshooting driving, a narrowed range of the applied displaying voltages causes a narrow dynamic range of display gray-level values in the liquid crystal panel, thus resulting in a decrease in image contrast.
Thus, in the liquid crystal driving method shown in FIG. 12, in the case of transition to the highest gray-level value or the lowest gray-level value, the overshooting operation is impossible, causing the occurrence of the trail leaving phenomenon at time of displaying moving images. Also, in the conventional liquid crystal driving method shown in FIG. 14, part of the display gray-level values is assigned for the overshooting operation and, by using this part, the overshooting operation is performed at time of the transition to the highest gray level or lowest gray level, however, the decrease in the number of gray-level values causes a narrow dynamic range of display gray-level values of the liquid crystal panel and, as a result, image contrast is lowered.
To solve this problem, a liquid crystal display device is disclosed in Patent Reference 1 (Japanese Patent Application Laid-open No. 2003-172915) in which a liquid crystal panel is designed so that, in the voltage—transmittance characteristic, the transmittance show its extreme value at a voltage exceeding the highest gray-level value and, by setting a voltage for overshooting driving to be higher than the voltage showing the extreme value of the transmittance, the rising state of the liquid crystal display device is improved and in which, at time of the overshooting operation for the transition to the highest gray-level displaying, the transmittance of the liquid crystal panel first reaches its extreme value and then transmittance level corresponding to the overshooting voltage.
However, an original purpose of the overshooting driving operation is to solve the problem that, due to viscosity of a liquid crystal substance, a change in its optical transmittance through a liquid crystal panel does not keep track of a change in voltages applied to a liquid crystal panel. Therefore, if the transmittance of the liquid crystal substance changes by keeping track of an applied voltage in such a way as described in the Patent Reference 1, the overshooting driving is not necessary. Moreover, in a normally-black liquid crystal panel such as an IPS (In Plane Switching)-type liquid crystal panel, when a voltage exceeding the maximum gray-level voltage is applied to its panel, the transmittance of the liquid crystal substance takes its extreme value. If a liquid crystal molecule is made to move up to a state in which the extreme value of the transmittance is exceeded, a liquid crystal molecule with a state in which the liquid crystal substance rotates in a reverse direction becomes stable at an end of a pixel structure of the liquid crystal element or a like, thus resulting in a defect of a tinny luminous dot within a pixel, color persistence on a screen and/or lowering in luminance. Therefore, application of the overshooting driving operation to the liquid crystal panel is impossible.
Also, another liquid crystal display device is disclosed in Patent Reference 2 (Japanese Patent Application Laid-open No. 2004-061692) in which a liquid crystal controller LSI (Large Scale Integrated Circuit) to control displaying of a liquid crystal panel includes a display control circuit having image memory to store image data input from an outer signal source, a liquid crystal driving voltage generating circuit having a reference voltage calibrating circuit and a data electrode driving circuit to supply image data stored in an image memory to a data electrode, wherein the first common grounding lines including grounding lines of the reference voltage calibrating circuit contained in the liquid crystal driving circuit and grounding lines of a scanning electrode circuit to receive a voltage from the liquid crystal driving circuit and of the data electrode driving circuit and the second grounding lines including a grounding line of image memory are arranged in a separated manner in the liquid crystal controller LSI so that degradation of display quality caused by noises and changes in voltage can be avoided. However, the technology disclosed in the Patent Reference 2 is the invention related to configurations of the reference voltage generating section in the liquid crystal controller LSI and is not related directly to the present invention.
Also, a flat panel displaying method and a flat panel display device are disclosed in Patent Reference 3 (Japanese Patent Application Laid-open No. Hei 07-334131) in which, through processing by an FRC, when still images are displayed, the first and second luminance being different from each other are sequentially assigned to display data that should be displayed as halftone display in predetermined number of pixels in a line and second and first luminance being opposite in order are sequentially assigned to display data that should be displayed as halftone display in a subsequent frame and, when moving images are displayed, the first luminance is assigned to display data that should be displayed as halftone display in pixels in an absolute position on a predetermined display screen in a frame and the second luminance is assigned in a subsequent frame, thereby preventing the occurrence of a flicker in the still images and making it possible to achieve correct gray-level display. However, the technology disclosed in the Patent Reference 3 is related to the processing by the FRC itself and is not directly contributable to the solution of problems associated with the conventional liquid crystal driving method.